1. Field of the Invention
The present invention relates to a line pressure control system for a shift control of an automatic transmission and, more particularly, to a line pressure control system for an automatic transmission, which is capable of properly controlling the line pressure even during a change in the engine load.
2. Description of Related Art
An automatic transmission, which determines a power transmission path (hence the gear ratio) of a gear transmission system by selectively engaging a plurality of friction elements (e.g., clutches, brakes) is usually designed so that the gear ratio can be changed by switching of the friction elements to be engaged. Since the engagement of the friction elements is achieved by selective supply of common line pressure, it is important to control the line pressure to a value according to an input torque of the automatic transmission, in order to avoid various potential problems.
For example, if the line pressure is insufficient with respect to the input torque of the transmission, the friction element tends to slip thereby causing a loss of power transmission besides degrading the durability of the friction element, while degrading the shift quality in that the actual shift is felt as being retarded. On the contrary, if the line pressure is excessively high with respect to the input torque of the transmission, the drive load of the oil pump for generating the line pressure becomes unnecessarily high, thereby deteriorating the fuel consumption of the engine and/or degrading the shift quality in that a large shift shock occurs upon shift operation.
In an attempt to overcome these problems, there have been proposed various methods for controlling the line pressure to a value according to the input torque of the automatic transmission. Thus, for example, JP 9-303541A discloses a shift control system wherein the output torque of the engine at the front stage of the automatic transmission is estimated from the engine rotational speed and the engine load (throttle valve opening) as taught by JP 3-41256A, and the input torque of the automatic transmission is calculated from the estimated engine torque and the operation state of a torque converter, so as to be used as the basis of the line pressure control.
According to the known shift control system described above, when the driver operates the accelerator pedal so as to change the engine load state, the estimation of the engine torque may not always be exact, thereby causing inaccurate line pressure control and resultant excess or deficiency thereof. In view of such additional problem, for example, JP 4-366059A proposes a line pressure control system, which estimates the engine torque when the accelerator pedal is depressed, by judging the engine load state from the throttle valve opening, but otherwise estimates the engine torque by judging the engine load state from the intake air amount or manifold vacuum of the engine. In other words, the known line pressure control system uses different parameters for estimation of the engine torque, which are optimum in terms of accuracy, for a condition in which the accelerator pedal is depressed and for other traveling conditions, respectively.
However, whichever parameter is used for estimation of the engine torque, the estimation necessarily has an estimation response delay with respect to an actual change in the engine torque, such that the instantaneous estimation value of the engine torque does not coincide with the actual engine torque. For this reason, the line pressure controlled based on the input torque of the automatic transmission, which in turn is calculated from the engine torque estimation value, tends to become excessive or deficient with respect to the actual transmission input torque. Thus, the known line pressure control system cannot avoid the above-mentioned problems that the drive load of the oil pump becomes unnecessarily high, thereby deteriorating the fuel consumption of the engine, and/or causing a large shift shock upon shift operation, besides an increased transmission loss, degradation of the durability of the friction element, and retarded feeling of the actual shift.
It is therefore a general object of the present invention to provide an improved line pressure control system for an automatic transmission, which eliminates the above-mentioned problems of the prior art.
It is a specific object of the present invention to provide an improved line pressure control system for an automatic transmission, in which the calculated value of the transmission input torque is adequately corrected when the engine load is decreased, so as to eliminate the excess of line pressure generated by the delay of the engine torque estimation with respect to the decrease in actual engine torque, and thereby avoid deterioration of the fuel consumption due to unnecessary increase in the oil pump drive load, and mitigate unpleasant shift shock.
According to the present invention, there is provided a line pressure control system for an automatic transmission wherein a shift range is determined by selectively engaging friction elements by means of a line pressure controlled according to an input torque of said automatic transmission, comprising:
estimation means for estimating an engine output torque at the front stage of said automatic transmission from the operating condition;
calculation means for calculating the input torque of said automatic transmission from the estimated engine output torque and the operation state of a torque converter; and
correction means for decreasing the calculated input torque of said automatic transmission during a predetermined time corresponding to an estimation response delay of the engine output torque after said engine is placed in a no-load state, so as to control said line pressure.
With the line pressure control system according to the present invention, the line pressure for selectively engaging the friction elements to determine the shift range is controlled by estimating the output torque of the engine from the operating condition, calculating the input torque of the automatic transmission from the estimated engine torque and the operation state of the torque converter, and decreasing the calculated input torque of said automatic transmission during a predetermined time corresponding to an estimation response delay of the engine output torque after said engine is placed in a no-load state, so as to control said line pressure.
Therefore, according to the present invention, when the engine load is decreased, the calculated value of transmission input torque is decreased so as to eliminate the excess of line pressure generated by a delay of the engine torque estimation with respect to the decrease in the actual engine torque. It is thus possible to avoid deterioration of the fuel consumption due to unnecessary increase in the oil pump drive load, and also to mitigate unpleasant shift shock.
It is preferred that, during a predetermined time corresponding to the estimated response delay of the engine output torque, after the engine load has been transferred to a sudden increase state in which the increase rate of the load of the engine is not lower than a predetermined value, the input torque of said automatic transmission at the time of the highest engine torque that can be generated under the increased engine load is used for said line pressure control. In this instance, when the engine load is increased, the highest engine torque that can be generated under the increased engine load is used in place of the engine torque estimation value to carry out line pressure control. Therefore, it is possible to eliminate deficiency of the line pressure due to the delay of the engine torque estimation with respect to the increase in the actual engine torque, and to thereby solve the problems of the transmission loss and/or deterioration in terms of the durability or retarded sense of the shift operation.
In addition to the above, it is preferred that the predetermined rate relating to the increase rate of the engine load is varied according to the hydraulic oil temperature of the automatic transmission, and the predetermined rate is decreased as the hydraulic oil temperature decreases so as to enhance sensitivity of judgment for the engine load sudden increase state. In a low temperature condition of the hydraulic oil of the automatic transmission, the viscosity of the hydraulic oil is high and the transmission capacity of the torque converter is higher than at a high temperature condition. Therefore, the increase in actual engine torque caused by the increase in the engine load is easily transmitted to the transmission, and the above-mentioned problem due to the relative estimation delay of the engine torque becomes marked. Since, however, the sensitivity of engine load sudden increase judgment can be enhanced at a low temperature condition, in the manner described above, it is possible to readily perform the judgment of the engine load increase state in which the line pressure control should be carried out, notwithstanding the low temperature condition. It is therefore possible to effectively eliminate the above-mentioned problem caused by the estimation delay of engine torque, and thereby positively achieve the line pressure control at any oil temperature condition.
It is further preferred that the line pressure control based on the input torque of the automatic transmission at the time of the highest engine torque is performed only in a high rotation region in which the engine rotational speed is not lower than a predetermined value. In this instance, it is possible to effectively perform the line pressure control only in the high rotation region of the engine, in which a change in the actual engine torque is rapid and the problem caused by the relative estimation delay of engine torque becomes noticeable.
It is alternatively preferred that the line pressure control based on the input torque of said automatic transmission at the time of said highest engine torque is performed only in a high vehicle speed region in which a vehicle speed is not lower than a predetermined vehicle value. In this instance, it is possible to effectively perform the line pressure control in the high vehicle speed region, in which a change in the actual engine torque is rapid and the problem caused by the relative estimation delay of engine torque becomes noticeable.